Intense visible photoluminescence in Ba„Zr0.25Ti0.75

APPLIED PHYSICS LETTERS 90, 011901 共2007兲
Intense visible photoluminescence in Ba„Zr0.25Ti0.75…O3 thin films
L. S. Cavalcantea兲 and M. F. C. Gurgel
Laboratório Interdisciplinar de Eletroquímica e Cerâmica, Departamento de Química,
Universidade Federal de São Carlos, P.O. Box 676, 13565-905 São Carlos, São Paulo, Brazil
A. Z. Simões, E. Longo, and J. A. Varela
Laboratório Interdisciplinar em Cerâmica, Departamento de Físico-Química, Instituto de Química,
Universidade Estadual Paulista, P.O. Box 355, 14801-907 Araraquara, São Paulo, Brazil
M. R. Joya and P. S. Pizani
Departamento de Física, Universidade Federal de São Carlos, P.O. Box 676, 13565-905 São Carlos,
São Paulo, Brazil
共Received 1 September 2006; accepted 25 November 2006; published online 2 January 2007兲
Photoluminescence at room temperature in Ba共Zr0.25Ti0.75兲O3 thin films was explained by the degree
of structural order-disorder. Ultraviolet-visible absorption spectroscopy, photoluminescence, and
first principles quantum mechanical measurements were performed. The film annealed at 400 ° C for
4 h presents intense visible photoluminescence behavior at room temperature. The increase of
temperature and annealing time creates 关ZrO6兴 – 关TiO6兴 clusters in the lattice leading to the trapping
of electrons and holes. Thus, 关ZrO5兴 – 关TiO6兴 / 关ZrO6兴 – 关TiO6兴 clusters were the main reason for the
photoluminescence behavior. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2425013兴
Photoluminescence 共PL兲 is a powerful tool to discover
the energy levels of materials when used in combination with
absorption and PL excitation measurements. PL provides
fundamental information of the structural degree within the
band gap of materials. Much interest has been focused on the
photoluminescence of disordered or nanostructured material,
since this phenomenon was first observed in porous silicon at
room temperature.1 In this context, the development of new
environmentally friendly materials is of fundamental importance. ABO3 perovskites 共A = Ba, Ca, Sr and B = Ti兲 have
been the focus of research aiming at their potential for photoluminescence properties.2–5 Kan et al.6 reported that the
main cause for PL emission at room temperature in polycrystalline SrTiO3 is oxygen deficiency. However, there are several papers explaining the favorable conditions for PL emission in materials presenting an order-disorder degree.7–9
Recently, Ba共ZrxTi1−x兲O3 共BZT兲 has been chosen as an alternative material to 共Ba, Sr兲TiO3 in the fabrication of ceramic
or thin films because the Zr4+ ion is chemically more stable
than the Ti4+ ion.10,11 However, its PL properties have not yet
been reported.
In this letter, we present the role of order-disorder to
explain the intense visible photoluminescence of
Ba共Zr0.25Ti0.75兲O3 共BZT兲 thin films at room temperature. The
films annealed at 400 ° C for different times and at 700 ° C
for 2 h were characterized by ultraviolet-visible 共UV-vis兲 absorption spectroscopy, PL, and quantum mechanical calculation measurements. This dual approach renders a plausible
quantitative description of BZT thin film behavior under laser excitation and an interesting correlation with experimental results. Quantum mechanical calculations have been carried out with the CRYSTAL98 package,12 which is based on
both density functional and Hartree-Fock methods. The
gradient-corrected correlation functional by Lee et al.13 and
Becke14 was used, combined with the Becke3 exchange
functional, B3LYP. The atomic centers have been described
a兲
Electronic mail: [email protected]
by all electron basis sets 9763-311共d631兲G for Ba,15 976
-31共d62兲G* for Zr,15 86-411共d31兲G for Ti,16 and 6-31G*
for O.16
BZT thin films were prepared by the soft chemical
method, as described in the following article.17 The viscosity
of the resulting solution was adjusted to 13 mPa s, controlling the water content using a Brookfield viscosimeter. The
polymeric solution was spin coated on the substrates by a
commercial spinner operating at 7500 rpm for 20 s 共spin
coater KW-4B, Chemat Technology兲. Such solution was deposited onto the Pt/ Ti/ SiO2 / Si substrates via a syringe filter
to avoid particulate contamination. After deposition, the
films were kept on a hot plate at 150 ° C in air ambient for
10 min to remove residual solvents. The films were annealed
at 400 ° C for 1, 2, 4, 8, and 16 h and at 700 ° C for 2 h with
a heating rate of 3 ° C / min in oxygen atmosphere.
The UV-vis absorption spectra of the optical absorbance
for disordered BZT thin films and crystalline BZT film were
taken at room temperature using Cary 5G equipment. The PL
spectra of the thin films were taken with a U-1000 JobinYvon double monochromator coupled to a cooled GaAs photomultiplier and a conventional photon counting system. The
514.5 nm exciting wavelength of an argon ion laser was
used, with the laser’s maximum output power kept at
30 mW. A cylindrical lens was used to prevent the sample
from overheating. The slit width used was 100 ␮m. All measurements were taken at room temperature.
Figure 1 illustrates the UV-vis spectral dependence of
absorbance for BZT thin films annealed at 400 ° C for different times and annealed at 700 ° C for 2 h in oxygen atmosphere. The optical band values obtained from the UV-vis
spectroscopy is shown in Fig. 1.
The exponential optical absorption edge and the optical
band gap are controlled by the structural order-disorder degree in the BZT film lattice. The increase of band gap can be
ascribed to the reduction of defects or impurities that give
rise to intermediary energy levels in the band gap region of
disordered BZT films. The films annealed at 400 ° C for 1, 2,
0003-6951/2007/90共1兲/011901/3/$23.00
90, 011901-1
© 2007 American Institute of Physics
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011901-2
Appl. Phys. Lett. 90, 011901 共2007兲
Cavalcante et al.
FIG. 1. UV-vis absorbance spectra for the BZT thin films annealed at
400 ° C for 共a兲 1 h, 共b兲 2 h, 共c兲 4 h, 共d兲 8 h, and 共e兲 16 h. Inset shows the
BZT film annealed at 700 ° C for 共f兲 2 h in oxygen atmosphere.
4, 8, and 16 h showed a similar spectral dependence to that
found in amorphous semiconductors such as silicon and insulators 关see Figs. 1共a兲–1共e兲兴, while the BZT film annealed at
700 ° C showed a typical band for crystalline materials. In
addition, in the high energy region of the absorbance curve
关see Fig. 1共f兲兴, the optical band gap energy is related to the
absorbance and to the photon energy by Wood and Tauc.18 In
the disordered films, the absorbance measurements suggest a
nonuniform band gap structure with a tail of localized states.
The optical energy band gap is related to the absorbance and
to the photon energy by Eq. 共1兲:
2
h␯␣ ⬀ 共h␯ − Eopt
g 兲 ,
共1兲
where ␣ is the absorbance, h is the Planck constant, ␯ is the
frequency, and Eopt
g is the optical band gap.
Table I illustrates the charge variations for each 关TiO6兴
and 关ZrO5兴, after displacement of Zr to deformation of
Ba共Zr0.25Ti0.75兲O3, and the charge difference between the individual clusters of periodic models for theoretical gap energies. Theoretical calculations indicate that the atom generated intermediary energy levels in the band gap, increasing
the trapping of electrons and holes. The trapping of charges
is the condition for a good PL emission. This accordance
between experimental and theoretical gap energy results illustrates the reliability of our distorted model.
Figure 2 illustrates the PL spectra recorded at room temperature for the BZT thin films annealed at 400 ° C for different times and annealed at 700 ° C for 2 h in oxygen atmo-
FIG. 2. PL spectra of BZT thin films annealed at 400 ° C for 共a兲 1 h, 共b兲 2 h,
共c兲 4 h, 共d兲 8 h, and 共e兲 16 h and 共f兲 annealed at 700 ° C for 2 h in oxygen
atmosphere. Inset shows experimental Eg共exp兲 and theoretical Eg共theo兲 gap energies for the films annealed at 400 ° C for different times.
sphere. The excitation line of argon ion laser was 514.5 nm
共⬵2.41 eV兲. The PL emission is associated with the structural order-disorder degree. A broad intense luminescence in
the visible region 共yellow兲 with a maximum at about 568 nm
was observed. The emission band profile is a typical multiphonon process, i.e., a system in which the relaxation occurs by several paths, involving the participation of numerous levels within the perovskite band gap.
The presence of several delocalized energy levels in the
band gap and the structural disorder are responsible for the
weak PL behavior observed in Figs. 2共a兲 and 2共b兲. The increase of annealing time reduces the oxygen vacancies and
decrease the structural disorder, due to the presence of
关ZrO5兴 – 关TiO6兴 clusters leading to an increase of PL intensity
关Fig. 2共c兲兴. Fig. 2共d兲 and 共e兲 illustrates that the PL intensity
reduces with the increase of annealing time due the formation of the 关关ZrO6兴 – 关TiO6兴 clusters as can be observed by
the increase of optical gap 共see inset in Fig. 2兲. When the
annealing temperature is increased, ordered 关ZrO6兴 – 关TiO6兴
clusters are obtained and the charge transference between
them 关Figs. 1共f兲 and 2共f兲兴 is inhibited. Experimental and theoretical band gap results strongly indicate that PL is directly
linked to the optical tail existing in the disordered films 共see
inset in Fig. 2兲. Absorbance measurements associated with
the PL properties of noncrystalline BZT thin films lead to a
nonuniform band gap structure with a tail of delocalized levels and mobile edges. The nature of these exponential optical
TABLE I. Charge variations for the 关TiO6兴, 关ZrO5兴, and 关TiO6兴 – 关ZrO5兴 clusters and experimental gap energy
Eg共exp兲 in axis x 共Fig. 1兲 and theoretical gap energy Eg共theo兲 for Ba共Zr0.25Ti0.75兲O3 thin films.
Displacement
in axis z 共Å兲
Charge of the
关TiO6兴 共e−兲
Charge of the
关ZrO5兴 共e−兲
Charge difference
关TiO6兴 – 关ZrO5兴 共e−兲
Eg共exp兲
共eV兲
Eg共theo兲
共eV兲
0.75
0.70
0.50
0.40
0.20
0.00
−2.37
−2.35
−2.26
−2.20
−2.06
−1.93
−1.98
−1.97
−1.90
−1.85
−1.74
−1.60
−0.39
−0.38
−0.36
−0.35
−0.32
−0.33
2.20
2.27
2.53
2.65
2.93
3.78
2.23
2.32
2.50
2.62
2.94
3.70
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011901-3
Appl. Phys. Lett. 90, 011901 共2007兲
Cavalcante et al.
FIG. 3. Wide band model: 共a兲 before excitation, 共b兲 excitation→ formation
of STE, and 共c兲 after excitation→ recombination of e− and h0.
edges and tails is probably associated with defects promoted
by the disordered structure.
A wide band model of photoluminescence process in
Ba共Zr0.25Ti0.75兲O3 thin films is shown in Fig. 3. The localized
levels are energetically distributed being able to excite the
trapped electrons 关see Fig. 3共a兲兴. According to the hypothesis
of Korzhik et al.19 there are vacant localized states linked to
local defects such as oxygen vacancies in the band gap above
the valence band and below the conduction band.6 In the
model of Leonelli and Brebner20 some electrons are promoted to the conduction band by absorption of photon, creating small polarons. The polarons interact with holes
trapped in the crystal 共defects or impurities兲 and form selftrapped excitons 共STEs兲 that contribute to the visible PL
emission 关see Fig. 3共b兲兴. The short and intermediate range
structural defects generate localized states in the band gap
and inhomogeneous charge distribution in the cell, thus allowing the trapping of electrons. Furthermore, it will also
leave an electron hole that can flow as current exactly like a
physical charged particle. Figure 3共c兲 describes the recombination process in which an electron of conduction band loses
its energy and reoccupies the energy levels of an electron
hole 共h0兲 in the valence band.
This model suggests that the increase of annealing time
reduces the surface, creating electron-captured oxygen’s vacancies, according to Eq. 共2兲:
1
共ZrO5 · Vzo兲 + O2 → ZrO2 ,
2
共2兲
*
z
·
··
where VO
= VO
, VO
, or VO
.
z
兲 cluster acts as hole traps,
In the complex, 共ZrO5 · VO
z
while the VO vacancy tends to trap electrons 共z = · or ··兲 or
holes 共z = *兲. Previous studies of our group assume the existence of TiO6 and distorted ZrO6 clusters in lattice before
complete film crystallization. A random mixture of TiO6 and
z
兲 complex octahedral linked by barium ions is
共ZrO5 · VO
found in this case. The displacements performed in axis z for
Zr atom, such as breaking of the bonds 共Zr–O兲 and formation
z
, give the band gap energy approaching experiof vacancy VO
mental data. In BZT, the transport properties such as electri-
cal conductivity and photoluminescence are governed by
electrons, as well as some phenomena are related to holes.
In conclusion, intense visible photoluminescence
共around 568 nm兲 was observed for the Ba共Zr0.25Ti0.75兲O3
thin films and is caused by structural defects such as
关ZrO5兴 – 关TiO6兴 clusters. The oxygen vacancies play an important role on the PL emission via a recombination of elecz
兲 with photoexcited holes in
trons in oxygen vacancies 共VO
the valence band. Our theoretical models are consistent with
the experimental data and confirm that the localized levels in
the band gap are controlled by the order-disorder degree in
the lattice.
The authors gratefully acknowledge the financial support
of the Brazilian financing agencies CAPES, CNPq, and
FAPESP.
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